a little weigh-in from another climber making his own gear:
i would go with a double line of stitching in a simple seam arrangement. French seams are for concealing the raw edges either 1) for aesthetic reasons (those frenchies and their style...) or 2) to prevent raveling, i.e., fraying of the edges where the individual threads of the fabric start to separate. In your case the seam is inside where it won't be seen, and you can prevent raveling by lightly searing your fabric edges with a candle.
likewise i think topstitching is unnecessary. in your case there is not a lot of tension perpendicular to your seam so you don't really need to distribute it across multiple rows of stitching.
personally i would use two lines of stitching separated by 1/8" or less. this way if the first row, which is indeed taking all of the stress on the seam, fails, there is a backup seam and you won't even notice the failure.